Cascode transistor and method of controlling cascode transistor

1. A cascade transistor comprising: a first switch; a second switch that has a withstand voltage higher than that of the first switch and is cascade coupled to a drain of the first switch; and a circuit in which a third switch and a capacitor are coupled in series with each other and that is provided between a connection node and a source of the first switch, the connection node being a node at which the first switch and the second switch are coupled to each other.

2. The cascade transistor according to claim 1 , wherein the third switch has an external connection terminal that allows the third switch to be switched from the outside.

3. The cascade transistor according to claim 2 , wherein a signal that turns the third switch on is input to the external connection terminal and the capacitor is electrically coupled to the connection node before a gate signal of the first switch is turned off.

4. The cascade transistor according to claim 2 , wherein a signal that turns the third switch off is input to the external connection terminal and the capacitor and the connection node are electrically coupled isolated from each other before a gate signal of the first switch is turned on.

5. The cascade transistor according to claim 1 , wherein the capacitor has a capacitance that changes an oscillation period of a drain voltage that oscillates due to a parasitic inductance coupled to a source of the cascade transistor and a gate-source capacitance so that the drain voltage is smallest at a timing at which the gate of the cascode transistor is turned on.

6. The cascode transistor according to claim 1 , wherein the capacitor is a voltage variable capacitor and has a control terminal through which its capacitance is changed.

7. The cascode transistor according to claim 1 , wherein the circuit, in which the capacitor and the third switch are coupled in series with each other, is provided in a plurality, the circuits being coupled in parallel between the connection node and the source of the first switch.

8. The cascode transistor according to claim 7 , further comprising a circuit that controls the circuits in which the capacitor and the third switch are coupled in series with each other.

9. The cascode transistor according to claim 1 , wherein the first switch is a normally-off-type MOS-FET and the second switch is a normally-on-type GaN-HEMT.

10. A method of controlling a cascode transistor that includes a first switch, a second switch that has a higher withstand voltage than the first switch and is cascode coupled to a drain of the first switch, and a circuit in which a third switch and a capacitor are coupled in series with each other and that is provided between a connection node and a source of the first switch, the connection node being a node at which the first switch and the second switch are coupled to each other, the method comprising: electrically coupling the capacitor to the connection node by turning the third switch on before a gate of the first switch is turned off.

11. The method of controlling a cascode transistor according to claim 10 , wherein the connection node and the capacitor are electrically isolated from each other by turning the third switch off before the gate of the first switch is turned on.

12. A semiconductor device comprising: a first switch that is formed on a semiconductor substrate or a second switch that has a higher withstand voltage than the first switch, is cascode coupled to the first switch and is formed on the semiconductor substrate; and a circuit in which a third switch and a capacitor are coupled in series with each other on the semiconductor substrate and that is provided between a connection node, at which the first switch and the second switch are coupled to each other, and a source of the first switch.